An image forming apparatus such as a color printer has a number of motors for driving various parts of the image forming apparatus such as a drum motor that drives a photoconductive drum. In order to produce high quality image, fluctuations in rotational speed of the motor need to be suppressed.
To suppress fluctuations in rotational speed of a motor such as the drum motor, a rotary encoder having an encoder sensor is fixed to a rotational shaft of the drum motor, or a rotational shaft of the photoconductive drum that rotates together with rotation of the drum motor. Based on an output signal of the encoder sensor, the rotational speed of the photoconductive drum or the drum motor is controlled.
In order to eliminate an error component of the output signal of the encoder sensor, which may be introduced by eccentricity of the encoder, two encoder sensors may be provided at predetermined positions of the encoder. By averaging output signals of the two encoder sensors, the error component attributable to eccentricity of the encoder can be eliminated. This technique of eliminating the error component using two encoder sensors, however, is not capable of eliminating an error component introduced when the encoder is deformed to be elliptical in shape.
In order to eliminate an error component attributable to elliptical deformation of the encoder, Japanese Patent Application Publication No. 2005-168280A discloses a rotary encoder having three encoders as illustrated in FIGS. 15A and 15B. FIG. 15A is a plan view of the rotary encoder, and FIG. 15B is a front view of the rotary encoder.
Referring to FIGS. 15A and 15B, the rotary encoder is implemented by a code wheel 110 that rotates around a rotational axis 160. Slits 110x are formed on a surface of the code wheel 110 in a circumferential direction of the code wheel 110. As illustrated in FIG. 15A, the code wheel 110 is provided with encoder sensors 320a, 320b, and 320c, which are arranged at 90-degree intervals in the circumferential direction of the code wheel 110.
A rotation measurement signal from which the error component attributable to eccentricity and the error component attributable to deformation are removed is obtained based on the output signals of the encoder sensors 320a, 320b, and 320c that are detected, and estimated values of the encoder sensors 320a and 320c that are estimated to be output as the phase is advanced by a certain degree with respect to the phase of the detected output signals are calculated using output signals of the encoder sensors 320a and 320c that were previously detected for a phase prior to the advanced phase.
However, this technique of removing the error component disclosed in Japanese Patent Application Publication No. 2005-168280A may not completely remove the error component superimposed on the output signals of the encoder sensors, because the estimated values of the encoder sensors that are necessary for error component elimination are calculated based on the output signals for the previous phase. Accordingly, fluctuations in rotational speed of the rotator may not be accurately detected.